Transmission system

ABSTRACT

A sun gear of a first planetary gear train is connected to an input shaft. A carrier  9  of the first planetary gear train is connected to a sun gear of a second planetary gear train and to a first pump/motor. A ring gear of the first planetary gear train is connected to a second pump/motor, and a ring gear of the second planetary gear train is connected to an output shaft. A first clutch is provided for engaging and disengaging a carrier of the second planetary gear train and the ring gear of the first planetary gear train with and from each other, and a second clutch is provided for engaging and disengaging the carrier of the second planetary gear train and a fixed end with and from each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Divisional Application of U.S. application Ser.No. 11/920,823, filed Nov. 20, 2007 now U.S. Pat. No. 7,972,236, whichis a U.S. National Phase Application under 35 USC 371 of InternationalApplication PCT/JP2006/308857, filed Apr. 27, 2006, the entire contentsof both of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a hydro-mechanical transmission systemhaving pumps/motors in combination with planetary gear trains or anelectro-mechanical transmission system having generators/motors incombination with planetary gear trains.

BACKGROUND ART

As hydraulic transmission systems, there have been known hydrostatictransmissions (HST) that convert all the input power from the engineinto hydraulic pressure for power transmission and hydro-mechanical(power split type) transmissions (HMT) that convert part of the inputpower into hydraulic pressure while mechanically transmitting the restof it. Of these types of transmissions, the latter transmissions (HMT)are advantageous over the former transmissions (HST) since HMTs onlyconvert part of the mechanical power into hydraulic, power and thereforeexert high mechanical power transmission efficiency. For this reason,HMTs are regarded as ideal transmissions for work vehicles subjected tosignificant load fluctuations such as bulldozers and wheel loaders andapplied to some of such vehicles.

In a typical hydro-mechanical transmission (HMT), its infinitelyvariable speed characteristics are attained by a planetary gear train.Specifically, the planetary gear train has three elements, i.e., a sungear, a carrier having planetary gears and a ring gear. Of these threeelements, the first and second elements are coupled to the input shaftand output shaft respectively and the third element is coupled to ahydraulic pump or hydraulic motor. The rotational speed of the hydraulicpump or hydraulic motor is varied to change the rotational speed of theoutput shaft.

HMTs are classified into two types. One is the output split type inwhich a first pump/motor is coupled to a planetary gear train and asecond pump/motor connected to the first pump through a hydrauliccircuit is coupled to the input shaft of the transmission system at aconstant rotation ratio. The other is the input split type in which afirst hydraulic pump or hydraulic motor is coupled to a planetary geartrain and a second hydraulic pump or hydraulic motor connected to thefirst one through a hydraulic circuit is coupled to the output shaft ofthe transmission system at a constant rotation ratio.

As a technique similar to HMTs, electro-mechanical transmissions (EMT)are known. EMTs use generators/motors in place of the pumps/motors usedin HMTs and convert part of mechanical power into electric power forpower transmission. A prior art technique associated with EMTs isdisclosed in Patent Document 1. The transmission system disclosed inthis document is an electro-mechanical transmission that has twoplanetary gear trains and two electric motors and is configured to beswitched by clutches to establish an input split mode to provide a lowspeed range and a compound split mode to provide a high speed range.

Patent Document: U.S. Pat. No. 6,478,705

FIG. 16 shows a schematic diagram of a transmission system constructedaccording to Patent Document 1. The transmission system 100 shown inFIG. 16 has an input shaft 103 that inputs power sent from an engine 101through a clutch 102; two planetary gearsets 104, 105 aligned coaxiallywith the input shaft 103; two generators/motors 106, 107 alignedcoaxially with the planetary gearsets 104, 105; an output shaft 108coupled to vehicle drive wheels (not shown) through a differentialgearset (not shown); and a pair of selectively engageable clutches 109,110. Each of the planetary gearsets 104, 105 is composed of a sun gear111 (112); a plurality of planetary gears 113 (114) in meshingengagement with the outer periphery of the sun gear 111 (112); a carrier115 (116) for supporting the shaft of the planetary gears 113 (114); anda ring gear 117 (118) in meshing engagement with the outer periphery ofthe planetary gears 113 (114).

Herein, the ring gear 117 of the planetary gearset 104 is connected tothe input shaft 103. The carrier 115 of the planetary gearset 104 andthe carrier 116 of the planetary gearset 105 are coupled to each otherby the output shaft 108 so as to be rotatable together with the outputshaft 108. The sun gears 111, 112 of the planetary gearsets 104, 105 arecoupled to the rotors 106 a, 107 a of the generators/motors 106, 107through sleeve shafts 119, 120, respectively, which are fitted on theoutput shaft 108. The clutch 109 is for connecting and disconnecting thering gear 118 to and from the fixed end, whereas the clutch 110 is forconnecting and disconnecting the ring gear 118 to and from the sleeveshaft 119. The stator 106 b of the generator/motor 106 and the stator107 b of the generator/motor 107 are electrically connected to a storagebattery 122 through an ECU (electronic control unit) 121.

In the transmission system 100, shifting between the input split modeand the compound split mode is effected at a vehicle speed at which therotational speed of the generator/motor 106 becomes zero (this vehiclespeed is hereinafter referred to as “mode switching point”). That is, ifthe current vehicle speed is within a vehicle speed range below the modeswitching point, the clutch 109 is engaged and the clutch 110 isdisengaged, thereby establishing the input split mode. On the otherhand, if the current vehicle speed is within a vehicle speed range abovethe mode switching point, the clutch 109 is disengaged whereas theclutch 110 is engaged thereby establishing the compound split mode.

Setting of the mode switching point may be carried out in threepatterns. In the first pattern, the mode switching point is set to avehicle speed Vb that is just a half of a vehicle speed Vd at which therotational speed of the generator/motor 107 becomes zero, as shown inFIG. 17( a). The second pattern is such that the mode switching point isset to a vehicle speed Vc that is higher than the vehicle speed Vb asshown in FIG. 17( b). In the third pattern, the mode switching point isset to a vehicle speed Va that is lower than the vehicle speed Vb asshown in FIG. 17( c). It should be noted that FIGS. 17( a) to 17(c) eachshow changes in the rotational speeds of the generators/motors 106, 107in cases where the vehicle is accelerated in the forward direction withthe rotational speed of the engine 101 being kept constant. In FIGS. 17(a) to 17(c), vehicle speed is plotted on the abscissa and the rotationalspeeds of the generators/motors 106, 107 on the ordinate. Solid lineindicated by A represents the change of the rotational speed of thegenerator/motor 107 relative to vehicle speed and broken line indicatedby B represents the change of the rotational speed of thegenerator/motor 106 relative to vehicle speed.

At the speed at which the rotational speed of the generator/motor 106becomes zero (i.e., the mode switching point (Vb in FIG. 17( a); Vc inFIG. 17( b); and Va in FIG. 17( c)), the rotational speed of thegenerator/motor 106 is zero and therefore the engine power is notconverted into electric power, so that all of the engine power istransmitted to the output shaft 108 through the mechanical mechanismalone. This “mode switching point” is also called “low speed side directpoint”. Also, at the speed at which the rotational speed of thegenerator/motor 107 becomes zero in the speed range corresponding to thecompound split mode (i.e., Vd in FIGS. 17( a) to 17(c)), the enginepower is not converted into electric power but entirely transmitted tothe output shaft 108 mechanically. This speed is hereinafter referred toas “high speed side direct point”.

In the planetary gearset 104, the ring gear (the third element) 117 isconnected to the input shaft 103, the carrier (the second element) 115is to the output shaft 108, and the sun gear (the first element) 111 isto the rotor 106 a of the generator/motor 106. Therefore, if vehiclespeed (the rotational speed of the output shaft 108) linearly changeswith the rotational speed of the engine 101 being kept constant, therotational speed of the generator/motor 106 will linearly change in allthe modes, i.e., the input split mode and the compound split mode asindicated by broken line B in FIGS. 17( a) to 17(c). In other words, therotational speed of the generator/motor 106 is directly affected by therotational speed of the output shaft 108 throughout all the modes, i.e.,the input split mode and the compound split mode.

Therefore, in the first pattern shown in FIG. 17( a), the rotatingdirection of the generator/motor 106 in the vehicle speed range of zerovehicle speed to the vehicle speed Vb (when the input split mode isselected) differs from the rotating direction of the generator/motor 106in the vehicle speed range of the vehicle speed Vb to the vehicle speedVd (when the compound split mode is selected). In addition, therelationship between the rotational speed Na of the generator/motor 106at zero vehicle speed and the rotational speed Nb of the generator/motor106 at the vehicle speed Vd is represented by Na=Nb. In the secondpattern shown in FIG. 17( b), the rotating direction of thegenerator/motor 106 in the vehicle speed range of zero vehicle speed tothe vehicle speed Vc (when the input split mode is selected) differsfrom the rotating direction of the generator/motor 106 in the vehiclespeed range of the vehicle speed Vc to the vehicle speed Vd (when thecompound split mode is selected), and the relationship between therotational speed Nc of the generator/motor 106 at zero vehicle speed andthe rotational speed Nd of the generator/motor 106 at the vehicle speedVd is represented by Nc>Nd. In the third pattern shown in FIG. 17( c),the rotating direction of the generator/motor 106 in the vehicle speedrange of zero vehicle speed to the vehicle speed Va (when the inputsplit mode is selected) differs from the rotating direction of thegenerator/motor 106 in the vehicle speed range of the vehicle speed Vato the vehicle speed Vd (when the compound split mode is selected), andthe relationship between the rotational speed Ne of the generator/motor106 at zero vehicle speed and the rotational speed Nf of thegenerator/motor 106 at the vehicle speed Vd is represented by Ne<Nf.

DISCLOSURE OF THE INVENTION Problems that the Invention Intends to Solve

Each of the above direct points is the most efficient vehicle speed atwhich all of the power coming from the engine 101 is transmitted throughthe mechanical mechanism alone. For setting the mode switching point,the first pattern shown in FIG. 17( a) or the second pattern shown inFIG. 17( b) are suitably adopted in the case of vehicles such as busesand transport trucks which often travel at intermediate speeds and highspeeds, and the third pattern shown in FIG. 17( c) is suitably adoptedin the case of construction vehicles such as bulldozers and wheelloaders which often operate to work at low speeds and travel at highspeeds.

However, if the prior art transmission system 100 adopts the secondpattern shown in FIG. 17( b) for setting the mode switching point,imbalance occurs between the applicable rotational speed range (0 to Nc)of the generator/motor 106 in the input split mode and the applicablerotational speed range (0 to Nd) of the generator/motor 106 in thecompound split mode. This causes a problem that the generator/motor 106is required to have high capacity to exert sufficient ability (torque)even for relatively low rotational speed Nd (<Nc). As a result, alarge-sized, costly generator/motor has to be employed as thegenerator/motor 106. If the third pattern shown in FIG. 17( c) isadopted for setting the mode switching point in the transmission system100, imbalance occurs between the applicable rotational speed range (0to Ne) of the generator/motor 106 in the input split mode and theapplicable rotational speed range (0 to Nf) of the generator/motor 106in the compound split mode. This also requires the generator/motor 106to have high capacity to exert sufficient ability (torque) even forrelatively low rotational speed Ne (<Nf), resulting in use of alarge-sized, costly generator/motor as the generator/motor 106.

It is apparent that the same problem as described above occurs in thecase of hydro-mechanical transmissions (HMT) in which pumps/motors areused in place of the generators/motors 106, 107 of the prior arttransmission system 100 and part of the mechanical power is convertedinto hydraulic power for power transmission. Further, in the case ofHMTs, the rotating direction of the pump/motor corresponding to thegenerator/motor 106 when vehicle speed is zero is opposite to therotating direction when vehicle speed is Vd and therefore abi-directional pump/motor has to be used as this pump/motor.

The present invention is directed to overcoming the foregoing problemsand a primary object of the invention is therefore to provide ahydro-mechanical transmission system or electro-mechanical transmissionsystem that enables use of pumps/motors or generators/motors smaller incapacity compared to the prior art, irrespective of setting of the modeswitching point that is the criterion of shifting between the inputsplit mode and the compound split mode.

Means for Solving the Problem

In accomplishing the above object, there has been provided, inaccordance with a first aspect of the invention, a transmission systemcomprising an input shaft, an output shaft, a mechanical transmissionsection and a hydraulic transmission section, the mechanical andhydraulic transmission sections being interposed between the input shaftand the output shaft, the hydraulic transmission section including aplurality of pumps/motors which are connectable to each other through ahydraulic circuit,

wherein the mechanical transmission section has a first planetary geartrain and a second planetary gear train;

wherein the plurality of pumps/motors consist of a first pump/motor anda second pump/motor;

wherein a first element of the first planetary gear train is connectedto the input shaft, a second element of the first planetary gear trainis connected to a first element of the second planetary gear train andto the first pump/motor, a third element of the first planetary geartrain is connected to the second pump/motor, and a third element of thesecond planetary gear train is connected to the output shaft;

wherein a first clutch is provided for engaging and disengaging a secondelement of the second planetary gear train and the third element of thefirst planetary gear train with and from each other; and

wherein a second clutch is provided for engaging and disengaging thesecond element of the second planetary gear train and a fixed end withand from each other.

According to a second aspect of the invention, there is provided atransmission system comprising an input shaft, an output shaft, amechanical transmission section and a hydraulic transmission section,the mechanical and hydraulic transmission sections being interposedbetween the input shaft and the output shaft, the hydraulic transmissionsection including a plurality of pumps/motors which are connectable toeach other through a hydraulic circuit,

wherein the mechanical transmission section has a first planetary geartrain and a second planetary gear train;

wherein the plurality of pumps/motors consist of a first pump/motor, asecond pump/motor and a third pump/motor;

wherein a first element of the first planetary gear train is connectedto the input shaft, a second element of the first planetary gear trainis connected to a first element of the second planetary gear train andto the first pump/motor, a third element of the first planetary geartrain is connected to the second pump/motor, and a third element of thesecond planetary gear train is connected to the output shaft;

wherein a first clutch is provided for engaging and disengaging a secondelement of the second planetary gear train and the third element of thefirst planetary gear train with and from each other;

wherein a second clutch is provided for engaging and disengaging thesecond element of the second planetary gear train and a fixed end withand from each other; and

wherein a switching mechanism is provided for selectively connecting thethird pump/motor to either the first pump/motor side or secondpump/motor side.

According to a third aspect of the invention, there is provided atransmission system comprising an input shaft, an output shaft, amechanical transmission section and a hydraulic transmission section,the mechanical and hydraulic transmission sections being interposedbetween the input shaft and the output shaft, the hydraulic transmissionsection including a plurality of pumps/motors which are connectable toeach other through a hydraulic circuit,

wherein the mechanical transmission section has a first planetary geartrain and a second planetary gear train,

wherein the plurality of pumps/motors consist of a first pump/motor anda second pump/motor;

wherein a first element of the first planetary gear train is connectedto the input shaft, a third element of the first planetary gear train isconnected to a first element of the second planetary gear train and tothe first pump/motor, a second element of the first planetary gear trainis connected to the second pump/motor, and a third element of the secondplanetary gear train is connected to the output shaft;

wherein a first clutch is provided for engaging and disengaging a secondelement of the second planetary gear train and the second element of thefirst planetary gear train with and from each other; and

wherein a second clutch is provided for engaging and disengaging thesecond element of the second planetary gear train and a fixed end withand from each other.

According to a fourth aspect of the invention, there is provided atransmission system comprising an input shaft, an output shaft, amechanical transmission section and a hydraulic transmission section,the mechanical and hydraulic transmission sections being interposedbetween the input shaft and the output shaft, the hydraulic transmissionsection including a plurality of pumps/motors which are connectable toeach other through a hydraulic circuit,

wherein the mechanical transmission section has a first planetary geartrain and a second planetary gear train;

wherein the plurality of pumps/motors consist of a first pump/motor, asecond pump/motor and a third pump/motor;

wherein a first element of the first planetary gear train is connectedto the input shaft, a third element of the first planetary gear train isconnected to a first element of the second planetary gear train and tothe first pump/motor, a second element of the first planetary gear trainis connected to the second pump/motor, and a third element of the secondplanetary gear train is connected to the output shaft;

wherein a first clutch is provided for engaging and disengaging a secondelement of the second planetary gear train and the second element of thefirst planetary gear train with and from each other;

wherein a second clutch is provided for engaging and disengaging thesecond element of the second planetary gear train and a fixed end withand from each other; and

wherein a switching mechanism is provided for selectively connecting thethird pump/motor to either a first pump/motor side or second pump/motorside.

The transmission system according to the second or fourth aspect of theinvention preferably comprises a switching valve for effecting switchingsuch that a flow of pressure oil to the third pump/motor is constantlydirected in a fixed direction (a fifth aspect of the invention).

The transmission system according to any one of the first to fourthaspects of the invention is preferably configured such that the speedratio of a low speed side direct point at which a rotational speed ofthe second pump/motor becomes zero to a high speed side direct point atwhich a rotational speed of the first pump/motor becomes zero is set to3 to 4 (a seventh aspect of the invention).

According to an eighth aspect of the invention, there is provided atransmission system comprising an input shaft, an output shaft, amechanical transmission section and an electric transmission section,the mechanical and electric transmission sections being interposedbetween the input shaft and the output shaft, the electric transmissionsection including a plurality of generators/motors which are drivinglycontrolled by an inverter,

wherein the mechanical transmission section has a first planetary geartrain and a second planetary gear train;

wherein the plurality of generators/motors consist of a firstgenerator/motor and a second generator/motor;

wherein a first element of the first planetary gear train is connectedto the input shaft, a second element of the first planetary gear trainis connected to a first element of the second planetary gear train andto the first generator/motor, a third element of the first planetarygear train is connected to the second generator/motor, and a thirdelement of the second planetary gear train is connected to the outputshaft;

wherein a first clutch is provided for engaging and disengaging a secondelement of the second planetary gear train and the third element of thefirst planetary gear train with and from each other; and

wherein a second clutch is provided for engaging and disengaging thesecond element of the second planetary gear train and a fixed end withand from each other.

According to a ninth aspect of the invention, there is provided atransmission system comprising an input shaft, an output shaft, amechanical transmission section and an electric transmission section,the mechanical and electric transmission sections being interposedbetween the input shaft and the output shaft, the electric transmissionsection including a plurality of generators/motors which are drivinglycontrolled by an inverter,

wherein the mechanical transmission section has a first planetary geartrain and a second planetary gear train;

wherein the plurality of generators/motors consist of a firstgenerator/motor, a second generator/motor and a third generator/motor;

wherein a first element of the first planetary gear train is connectedto the input shaft, a second element of the first planetary gear trainis connected to a first element of the second planetary gear train andto the first generator/motor, a third element of the first planetarygear train is connected to the second generator/motor, and a thirdelement of the second planetary gear train is connected to the outputshaft;

wherein a first clutch is provided for engaging and disengaging a secondelement of the second planetary gear train and the third element of thefirst planetary gear train with and from each other;

wherein a second clutch is provided for engaging and disengaging thesecond element of the second planetary gear train and a fixed end withand from each other; and

wherein a switching mechanism is provided for selectively connecting thethird generator/motor to either a first generator/motor side or secondgenerator/motor side.

According to a tenth aspect of the invention, there is provided atransmission system comprising an input shaft, an output shaft, amechanical transmission section and an electric transmission section,the mechanical and electric transmission sections being interposedbetween the input shaft and the output shaft, the electric transmissionsection including a plurality of generators/motors which are drivinglycontrolled by an inverter,

wherein the mechanical transmission section has a first planetary geartrain and a second planetary gear train;

wherein the plurality of generators/motors consist of a firstgenerator/motor and a second generator/motor;

wherein a first element of the first planetary gear train is connectedto the input shaft, a third element of the first planetary gear train isconnected to a first element of the second planetary gear train and tothe first generator/motor, a second element of the first planetary geartrain is connected to the second generator/motor, and a third element ofthe second planetary gear train is connected to the output shaft;

wherein a first clutch is provided for engaging and disengaging a secondelement of the second planetary gear train and the second element of thefirst planetary gear train with and from each other; and

wherein a second clutch is provided for engaging and disengaging thesecond element of the second planetary gear train and a fixed end withand from each other.

According to an eleventh aspect of the invention, there is provided atransmission system comprising an input shaft, an output shaft, amechanical transmission section and an electric transmission section,the mechanical and electric transmission sections being interposedbetween the input shaft and the output shaft, the electric transmissionsection including a plurality of generators/motors which are drivinglycontrolled by an inverter,

wherein the mechanical transmission section has a first planetary geartrain and a second planetary gear train;

wherein the plurality of generators/motors consist of a firstgenerator/motor, a second generator/motor and a third generator/motor;

wherein a first element of the first planetary gear train is connectedto the input shaft, a third element of the first planetary gear train isconnected to a first element of the second planetary gear train and tothe first generator/motor, a second element of the first planetary geartrain is connected to the second generator/motor, and a third element ofthe second planetary gear train is connected to the output shaft;

wherein a first clutch is provided for engaging and disengaging a secondelement of the second planetary gear train and the second element of thefirst planetary gear train with and from each other;

wherein a second clutch is provided for engaging and disengaging thesecond element of the second planetary gear train and a fixed end withand from each other; and

wherein a switching mechanism is provided for selectively connecting thethird generator/motor to either a first generator/motor side or secondgenerator/motor side.

The transmission system according to any one of the eighth to eleventhaspects of the invention is preferably configured such that the speedratio of a low speed side direct point at which a rotational speed ofthe second generator/motor becomes zero to a high speed side directpoint at which a rotational speed of the first generator/motor becomeszero is set to 3 to 4 (a twelfth aspect of the invention).

Effects of the Invention

In cases where the transmission system of the first aspect is applied toa vehicle, shifting between the input split mode and a compound splitmode is effected on the basis of the vehicle speed (hereinafter referredto as “mode switching point”) at which the rotational speed of thesecond pump/motor becomes zero. More specifically, if the presentvehicle speed is within a vehicle speed range below the mode switchingpoint, the first clutch disengages the second element of the secondplanetary gear train from the third element of the first planetary geartrain, whereas the second clutch engages the second element of thesecond planetary gear train with the fixed end, whereby the input splitmode is established. At zero vehicle speed that provides the input splitmode, the rotational speed of the first pump/motor is zero and therotational speed Nx of the second pump/motor for a certain inputrotational speed is uniquely determined in accordance with the toothnumber ratio of the first planetary gear train. At a high speed sidedirect point that provides the compound split mode (i.e., when therotational speed of the first pump/motor is zero), the rotational speedNy of the second pump/motor for the certain input rotational speed isuniquely determined in accordance with the tooth number ratio of thefirst planetary gear train. Therefore, Nx=Ny and these modes can be madeequal to each other in terms of the rotational speed and rotatingdirection of the second pump/motor. Therefore, whatever vehicle speedthe mode switching point is set to, the applicable rotational speedrange of the second pump/motor in the input split mode and theapplicable rotational speed range of the second pump/motor in thecompound split mode can be evenly balanced, so that a pump/motor havingsmaller capacity than that of the prior art can be used as the secondpump/motor. In addition, since the rotating direction in the input splitmode is the same as in the compound split mode, a mono-directionalpump/motor can be used as the second pump/motor.

The transmission system of the second aspect has the same basicconfiguration as of the transmission system of the first aspect andtherefore the same operational effects as of the first aspect. Further,in the transmission system of the second aspect, the third pump/motorcan be selectively connected to the first pump/motor side and the secondpump/motor side to consistently assist the function of either the firstor second pump/motor. As a result, pumps/motors having smaller capacitycan be used for these pumps/motors.

In cases where the transmission system of the third aspect is applied toa vehicle, shifting between the input split mode and the compound splitmode can be effected, similarly to the first aspect, on the basis of thevehicle speed (mode switching point) at which the rotational speed ofthe second pump/motor becomes zero. More specifically, if the presentvehicle speed is within a vehicle speed range below the mode switchingpoint, the first clutch disengages the second element of the secondplanetary gear train from the second element of the first planetary geartrain, whereas the second clutch engages the second element of thesecond planetary gear train with the fixed end, whereby the input splitmode is established. At zero vehicle speed that provides the input splitmode, the rotational speed of the first pump/motor is zero and therotational speed Nx′ of the second pump/motor for a certain inputrotational speed is uniquely determined in accordance with the toothnumber ratio of the first planetary gear train. At the high speed sidedirect point that provides the compound split mode (i.e., when therotational speed of the first pump/motor is zero), the rotational speedNy′ of the second pump/motor for the certain input rotational speed isuniquely determined in accordance with the tooth number ratio of thefirst planetary gear train. Therefore, Nx′=Ny′ and these modes can bemade equal to each other in terms of the rotational speed and rotatingdirection of the second pump/motor. Therefore, whatever vehicle speedthe mode switching point is set to, the applicable rotational speedrange of the second pump/motor in the input split mode and theapplicable rotational speed range of the second pump/motor in thecompound split mode can be evenly balanced like the first aspect, sothat a pump/motor having smaller capacity than that of the prior art canbe used as the second pump/motor. In addition, since the rotatingdirection in the input split mode is the same as in the compound splitmode, a mono-directional pump/motor can be used as the secondpump/motor.

The transmission system of the fourth aspect has the same basicconfiguration as of the transmission system of the third aspect andtherefore the same operational effects as of the third aspect. Further,in the transmission system of the fourth aspect, the third pump/motorcan be selectively connected to the first pump/motor side and the secondpump/motor side to consistently assist the function of either the firstor second pump/motor. As a result, pumps/motors having smaller capacitycan be used for these pumps/motors.

Adoption of the configuration of the transmission system of the fifthaspect makes it possible to use an inexpensive mono-directionalpump/motor as the third pump/motor.

The transmission systems of the seventh and twelfth aspects have theeffect of providing excellent power transmission efficiency when appliedto a construction vehicle.

The transmission system of the eighth aspect is an electric-mechanicaltransmission system corresponding to the hydraulic-mechanicaltransmission system of the first aspect and therefore a compactgenerator/motor having smaller capacity than the prior art can be usedas the second generator/motor, similarly to the first aspect.

The transmission system of the ninth aspect is an electric-mechanicaltransmission system corresponding to the hydraulic-mechanicaltransmission system of the second aspect and therefore compactgenerators/motors having smaller capacity than the prior art can be usedas the generators/motors, similarly to the second aspect.

The transmission system of the tenth aspect is an electric-mechanicaltransmission system corresponding to the hydraulic-mechanicaltransmission system of the third aspect and therefore a compactgenerator/motor having smaller capacity than the prior art can be usedas the second generator/motor, similarly to the third aspect.

The transmission system of the eleventh aspect is an electric-mechanicaltransmission system corresponding to the hydraulic-mechanicaltransmission system of the fourth aspect and therefore compactgenerators/motors having smaller capacity than the prior art can be usedas the generators/motors, similarly to the fourth aspect.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic structural diagram of a transmission systemaccording to a first embodiment of the invention.

FIG. 2 is operating characteristic graphs of the transmission systemaccording to the first embodiment.

FIG. 3 is a schematic structural diagram of a transmission systemaccording to a second embodiment of the invention.

FIG. 4 is operating characteristic graphs of the transmission systemaccording to the second embodiment.

FIG. 5 is a diagram illustrating an embodiment in which a hydrauliccircuit is provided with a communication valve.

FIG. 6 is a schematic structural diagram of a transmission systemaccording to a third embodiment of the invention.

FIG. 7 is operating characteristic graphs of the transmission systemaccording to the third embodiment.

FIG. 8 is a schematic structural diagram of a transmission systemaccording to a fourth embodiment of the invention.

FIG. 9 is operating characteristic graphs of the transmission systemaccording to the fourth embodiment.

FIG. 10 is a schematic structural diagram of a transmission systemaccording to a fifth embodiment of the invention.

FIG. 11 is a schematic structural diagram of a transmission systemaccording to a sixth embodiment of the invention.

FIG. 12 is a schematic structural diagram of a transmission systemaccording to a seventh embodiment of the invention.

FIG. 13 is a schematic structural diagram of a transmission systemaccording to an eighth embodiment of the invention.

FIG. 14 is a schematic structural diagram of a transmission systemaccording to a ninth embodiment of the invention.

FIG. 15 is a schematic structural diagram of a transmission systemaccording to a tenth embodiment of the invention.

FIG. 16 is a schematic structural diagram of a transmission systemaccording to a prior art technique.

FIG. 17 shows graphs of set patterns for a mode switching point.

EXPLANATION OF REFERENCE NUMERALS

-   -   1, 1A to 1I: transmission system    -   2: engine    -   4: input shaft    -   5: first planetary gear train    -   6: second planetary gear train    -   7, 11: sun gear (first element)    -   8, 12: planetary gears    -   9, 13: carrier (second element)    -   10, 14: ring gear (third element)    -   16: first pump/motor    -   16A: first generator/motor    -   20: second pump/motor    -   20A: second generator/motor    -   25: output shaft    -   27: first clutch    -   28: fixed end    -   29: second clutch    -   30, 32: hydraulic pipeline (hydraulic circuit)    -   31: third pump/motor    -   31A: third generator/motor    -   37: third clutch    -   40: fourth clutch    -   43: switching valve    -   44, 44A: switching mechanism    -   50, 50A: controller

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the accompanying drawings, the transmission system ofthe invention will be hereinafter described according to preferredembodiments.

First Embodiment

FIG. 1 shows a schematic structural diagram of a transmission systemconstructed according to a first embodiment of the invention. The firstembodiment is associated with a transmission system applied to aconstruction vehicle such as bulldozers and wheel loaders.

The transmission system 1 of the first embodiment has an input shaft 4that inputs power from an engine 2 through a forward/reverse switchingmechanism 3. The transmission system 1 also includes a first planetarygear train 5 and a second planetary gear train 6 which are coaxiallyaligned with the input shaft 4.

The first planetary gear train 5 includes a sun gear 7 secured to theinput shaft 4; a plurality of planetary gears 8 in meshing engagementwith the outer periphery of the sun gear 7; a carrier 9 for supportingthe shaft of the planetary gears 8; and a ring gear 10 in meshingengagement with the outer periphery of the group of the planetary gears8. The second planetary gear train 6 includes a sun gear 11; a pluralityof planetary gears 12 in meshing engagement with the outer periphery ofthe sun gear 11; a carrier 13 for supporting the shaft of the planetarygears 12; and a ring gear 14 in meshing engagement with the outerperiphery of the group of the planetary gears 12.

A first gear 15 is integrally coupled to the carrier 9 of the firstplanetary gear train 5, and a second gear 18 secured to an output shaft17 of a first pump/motor 16 is in meshing engagement with the first gear15. A third gear 19 is integrally coupled to the ring gear 10 of thefirst planetary gear train 5, and a fourth gear 22 secured to an outputshaft 21 of a second pump/motor 20 is in meshing engagement with thethird gear 19. The carrier 9 of the first planetary gear train 5 and thesun gear 11 of the second planetary gear train 6 are coupled to eachother by means of a sleeve shaft 23 rotatably supported on the inputshaft 4. A fifth gear 24 is integrally coupled to the ring gear 14 ofthe planetary gear train 6 and a sixth gear 26 secured to an outputshaft 25 is in meshing engagement with the fifth gear 24.

The transmission system 1 is equipped with a first clutch 27 forconnecting and disconnecting the carrier 13 of the second planetary geartrain 6 to and from the ring gear 10 of the first planetary gear train 5and with a second clutch 29 for connecting and disconnecting the carrier13 of the second planetary gear train 6 to and from a fixed end 28. Thefirst pump/motor 16 and the second pump/motor 20 are connected to eachother through a hydraulic pipeline 30. The hydraulic pipeline 30 isprovided with a shuttle valve 46 whereas a pipeline 48 for connectingthe shuttle valve 46 to a tank 47 is provided with a relief valve 49.

The first pump/motor 16 and the second pump/motor 20 are both variabledisplacement hydraulic pumps/motors. The first pump/motor 16 has a firstpump/motor displacement control system 51 for controlling thedisplacement of the first pump/motor 16 in response to an instructionsignal from a controller 50. The second pump/motor 20 is also providedwith a second pump/motor displacement control system 52 for similarlycontrolling the displacement of the second pump/motor 20 in response toan instruction signal from the controller 50. The first clutch 27 andthe second clutch 29 are both hydraulically actuated clutches. The firstclutch 27 has a first clutch pressure control valve 53 for controllingthe clutch pressure of the first clutch 27 in response to an instructionsignal from the controller 50. The second clutch 29 has a second clutchpressure control valve 54 for similarly controlling the clutch pressureof the second clutch 29 in response to an instruction signal from thecontroller 50. Herein, the controller 50 is composed of a centralprocessing unit (CPU) for mainly executing a specified program; a readonly memory (ROM) for storing this specified program and various tables;a random access memory (RAM) that serves as a working memory necessaryfor executing the program; an input interface; and an output interface.

Reference is made to FIG. 2 and Table 1 to explain a case where vehiclespeed is accelerated from zero while keeping the input rotational speedtransmitted from the engine 2 to the input shaft 4 constant in thetransmission system 1 of the invention. FIG. 2 shows operatingcharacteristic graphs of the transmission system of this embodiment.Specifically, FIG. 2( a) is a graph showing changes in the rotationalspeed of each pump/motor relative to vehicle speed; FIG. 2( b) is agraph showing changes in the displacement of the first pump/motorrelative to vehicle speed; and FIG. 2( c) is a graph showing changes inthe displacement of the second pump/motor relative to vehicle speed.Table 1 shows the operating condition (ON (connected)/OFF (disconnected)state) of each clutch in relation to vehicle speed ranges. In FIG. 2(a), solid line represents changes in the rotational speed of the firstpump/motor relative to vehicle speed whereas dotted line representschanges in the rotational speed of the second pump/motor relative tovehicle speed.

TABLE 1 Vehicle Speed Range A Vehicle Speed Range B First Clutch OFF ONSecond Clulch ON OFF

The displacements of the pumps/motors 16, 20 are controlled according tovehicle speed as shown in FIGS. 2( b), 2(c), by means of the first andsecond pump/motor displacement control systems 51, 52, respectively,that operate in response to instruction signals from the controller 50.As a result, the rotational speeds of the pumps/motors 16, 20 vary asshown in FIG. 2( a). At a vehicle speed V2 which serves as the switchingpoint and at which the displacement of the first pump/motor 16 becomeszero, the first and second clutches 27, 29 are switched as shown inTable 1 by the first and second clutch pressure control valves 53, 54,respectively, that operate in response to instruction signals from thecontroller 50.

As shown in Table 1, in the vehicle speed range A not higher than thevehicle speed V2, the first clutch 27 is turned OFF to disengage thecarrier 13 of the second planetary gear train 6 from the ring gear 10 ofthe first planetary gear train 5, whereas the second clutch 29 is turnedON to engage the carrier 13 of the second planetary gear train 6 withthe fixed end 28, thereby establishing the input split mode. At thattime, the power of the engine 2 is input to the sun gear 7 of the firstplanetary gear train 5 and this power is output from the carrier 9 tothe sun gear 11 of the second planetary gear train 6 through the sleeveshaft 23. The power input to the sun gear 7 is transmitted from theplanetary gears 8 to the ring gear 10 and output through the third gear19 and the fourth gear 22 to the second pump/motor 20 that serves as apump. The power output to the second pump/motor 20 is transmittedthrough the hydraulic pipeline 30 to the first pump/motor 16 that servesas a motor. The rotational power of the first pump/motor 16 is outputfrom the output shaft 17 to the sun gear 11 of the second planetary geartrain 6 through the second gear 18, the first gear 15, the carrier 9 andthe sleeve shaft 23. In this way, the power output to the sun gear 11 ofthe second planetary gear train 6 is transmitted from the planetarygears 12 to the output shaft 25 through the ring gear 14, the fifth gear24 and the sixth gear 26 and becomes the rotational power of the outputshaft 25.

The first embodiment is associated with a case where the invention isapplied to a construction vehicle. In this embodiment, the displacementof the second pump/motor 20 is not set to zero when vehicle speed iszero, in order to allow the output shaft 25 to generate torque toproduce traction force even when vehicle speed is zero (see FIG. 2( c)).When vehicle speed is zero, the second pump/motor 20 performs pumpingoperation because its displacement is not zero, but the first pump/motor16 does not rotate because of zero vehicle speed, so that all pressureoil is released by the relief valve 49. As vehicle speed increases, therotational speed of the first pump/motor 16 increases and the releasedpressure oil decreases gradually. Specifically, in the first embodiment,even when vehicle speed is zero, torque can be generated on the outputshaft 25 through the hydraulic transmission section in accordance withthe set pressure of the relief valve 49.

As shown in Table 1, in a vehicle speed range B exceeding V2, the firstclutch 27 is turned ON to engage the carrier 13 of the second planetarygear train 6 with the ring gear 10 of the first planetary gear train 5,whereas the second clutch 29 is turned OFF to disengage the carrier 13of the second planetary gear train 6 from the fixed end 28, therebyestablishing the compound split mode. That is, the vehicle speed V2 is amode switching point at which shifting between the input split mode andthe output split mode is effected. The vehicle speed V2 is also the lowspeed side direct point at which the displacement of the firstpump/motor 16 is zero and engine power is all transmitted to the outputshaft 25 through the mechanical transmission section alone. It should benoted that, at vehicle speed V2, the rotational speed of the secondpump/motor 20 is zero and the rotational speed of the ring gear 10 ofthe first planetary gear train 5 is equal to that of the carrier 13 ofthe second planetary gear train 6. Therefore, a dog clutch of simplestructure can be used as the first clutch 27 and the second clutch 29.

In the vehicle speed range B, the power of the engine 2 is input to thesun gear 7 of the first planetary gear train 5 and this power is outputfrom the carrier 9 to the sun gear 11 of the second planetary gear train6 through the sleeve shaft 23. The power input to the sun gear 7 istransmitted from the carrier 9 to the first pump/motor 16 that serves asa pump, through the first gear 15 and the second gear 18. The poweroutput to the first pump/motor 16 is then transmitted through thehydraulic pipeline 30 to the second pump/motor 20 that serves as amotor. The rotational power of the second pump/motor 20 is transmittedfrom its output shaft 21 to the carrier 13 of the second planetary geartrain 6 through the fourth gear 22, the third gear 19, and the ring gear10. In this way, the power output to the sun gear 11 of the secondplanetary gear train 6 is transmitted to the ring gear 14 through theplanetary gears 12, whereas the power output to the carrier 13 of thesecond planetary gear train 6 is transmitted to the ring gear 14 throughthe planetary gears 12. These powers are combined at the ring gear 14and transmitted to the output shaft 25 through the fifth gear 24 and thesixth gear 26, becoming the rotational power of the output shaft 25.

At a vehicle speed V4, the rotational speed of the first pump/motor 16reaches zero while the displacement of the second pump/motor 20 becomeszero. At that time, the second pump/motor 20 runs idle with the firstpump/motor 16 being stopped, and no oil flows in the hydraulic pipeline30 so that no power is transmitted by oil pressure. Thus, vehicle speedreaches the high speed side direct point at which the power of theengine 2 is all transmitted through the mechanical transmission sectionalone.

In the transmission system 1 of the first embodiment, shifting betweenthe input split mode and the compound split mode is effected on thebasis of the vehicle speed at which the rotational speed of the secondpump/motor 20 becomes zero (i.e., the mode switching point).Specifically, if the present vehicle speed is in a vehicle speed rangebelow the mode switching point, the carrier 13 of the second planetarygear train 6 will be disengaged from the ring gear 10 of the firstplanetary gear train 5 by the first clutch 27, while engaging thecarrier 13 of the second planetary gear train 6 with the fixed end 28 bythe second clutch 29, whereby the input split mode will be established.In the input split mode in which vehicle speed is zero, the rotationalspeed of the first pump/motor 16 is zero and the rotational speed of thesecond pump/motor 20 for a certain input rotational speed is uniquelydetermined in accordance with the tooth number ratio of the firstplanetary gear train 5. At the high speed side direct point thatprovides the compound split mode (i.e., when the rotational speed of thefirst pump/motor 16 is zero), the rotational speed of the secondpump/motor 20 for the certain input rotational speed is uniquelydetermined in accordance with the tooth number ratio of the firstplanetary gear train 5. Specifically, as shown in FIG. 2( a), therotational speed of the second pump/motor 20 becomes −N1 at both zerovehicle speed and the high speed side direct point V4. Thus, the samerotational speed and same rotating direction can be set for the inputsplit mode and the compound split mode. Therefore, whatever vehiclespeed the mode switching point is set to, the applicable rotationalspeed range of the second pump/motor 20 in the input split mode and theapplicable rotational speed range of the second pump/motor 20 in thecompound split mode can be evenly balanced, so that a pump/motor havingsmaller capacity than the prior art can be used as the second pump/motor20. In addition, since the rotating direction in the input split modedoes not differ from the rotating direction in the compound split mode,a mono-directional pump/motor can be used as the second pump/motor 20.

Generally, in construction vehicles such as bulldozers and wheelloaders, the vehicles travel at vehicle speeds approximately three tofour times the vehicle speeds during construction operation. Forinstance, in the case of bulldozers, dozing operation is performed at aspeed of about 3 km/h whereas their maximum speed is about 11 km/h. Inthe case of wheel loaders, V-shape loading is performed at a speed ofabout 10 km/h whereas their maximum speed is about 35 km/h. The lowspeed side direct point and high speed side direct point are the vehiclespeeds at which the best transmission efficiency of the transmissionsystem can be obtained because the power of the engine 2 is alltransmitted through the mechanical mechanism at these points. Therefore,in the first embodiment, the mode switching point is set such that thelow speed side direct point (vehicle speed V2) corresponds to a vehiclespeed for construction operation whereas the high speed side directpoint (vehicle speed V4) corresponds to the maximum speed. In otherwords, the mode switching point is set so as to make the speed ratio(V4/V2) of the high speed side direct point to the low speed side directpoint be 3 to 4 (the same is applied to each of the followingembodiments). This has the effect of providing highly improvedefficiency. In order to attain improved efficiency in cases where thetransmission system 1 of the first embodiment is applied to vehiclessuch as buses and transport trucks which often travel at intermediateand high speeds, the mode switching point is set such that the speedratio (V4/V2) of the high speed side direct point to the low speed sidedirect point is 2 or less.

Second Embodiment

FIG. 3 is a schematic structural diagram of a transmission systemconstructed according to a second embodiment of the invention. In thesecond embodiment, parts substantially similar or corresponding to thoseof the first embodiment are identified by the same reference numeralsand a detailed description thereof is omitted herein.

In the transmission 1A of the second embodiment, a third pump/motor 31is connected to the hydraulic pipeline 30 through a hydraulic pipeline32. The third pump/motor 31 is provided with a third pump/motordisplacement control system 55 for adjusting the displacement of thethird pump/motor 31 in response to an instruction signal from thecontroller 50. Also, a first shaft 33 and a second shaft 34 are disposedin parallel with the input shaft 4.

A seventh gear 35 is secured to the first shaft 33 and an eighth gear 36is coupled to the first shaft 33 through a third clutch 37. A ninth gear38 is secured to the second shaft 34 and a tenth gear 39 is coupled tothe second shaft 34 through a fourth clutch 40. Meshingly engaged withthe seventh gear 35 are the tenth gear 39 and an eleventh gear 42 thatis secured to an output shaft 41 of the third pump/motor 31. A firstgear 15 is in meshing engagement with the eight gear 36 and the thirdgear 19 is in meshing engagement with the ninth gear 38. It should benoted that the gear mechanism 44 (hereinafter referred to as “switchingmechanism 44”) including the third clutch 37 and the fourth clutch 40corresponds to the “switching mechanism” of the invention.

The third clutch 37 and the fourth clutch 40 are both hydraulicclutches. The third clutch 37 is equipped with a third clutch pressurecontrol valve 56 for controlling the clutch pressure of the third clutch37 in response to an instruction signal from the controller 50 whereasthe fourth clutch 40 is similarly equipped with a fourth clutch pressurecontrol valve 57 for controlling the clutch pressure of the fourthclutch 40 in response to an instruction signal from the controller 50.

Reference is made to FIG. 4 and Table 2 to describe a case where vehiclespeed is accelerated from zero while keeping the input rotational speedtransmitted from the engine 2 to the input shaft 4 constant in thetransmission system 1A of the second embodiment. FIG. 4 shows operatingcharacteristic graphs of the transmission system of this embodiment.Specifically, FIG. 4(a) is a graph showing changes in the rotationalspeed of each pump/motor relative to vehicle speed; FIG. 4( b) is agraph showing changes in the displacement of the first pump/motorrelative to vehicle speed; and FIG. 4( c) is a graph showing changes inthe displacement of the second pump/motor relative to vehicle speed; andFIG. 4( d) is a graph showing changes in the displacement of the thirdpump/motor relative to vehicle speed. Table 2 shows the operatingcondition (ON (connected)/OFF (disconnected) state) of each clutch inrelation to vehicle speed ranges. In FIG. 4( a), solid line, dottedline, alternate long and short dash line represent changes in therotational speeds of the first, second and third pumps/motors,respectively, relative to vehicle speed.

TABLE 2 Vehicle Vehicle Vehicle Vehicle Speed Speed Speed Speed Range ARange B Range C Range D First Clutch OFF OFF ON ON Second Clutch ON ONOFF OFF Third Clutch ON OFF OFF ON Fourth Clutch OFF ON ON OFF

As shown in FIGS. 4( b), 4(c), 4(d), the displacements of thepumps/motors 16, 20, 31 are controlled by the pump/motor displacementcontrol systems 51, 52, 55 respectively in response to instructionsignals from the controller 50 according to vehicle speeds. As a result,the rotational speeds of the pumps/motors 16, 20, 31 vary as shown inFIG. 4( a). At the vehicle speed V2 at which the displacement of thefirst pump/motor 16 reaches zero and the vehicle speeds V1, V3 at whichthe displacement of the third pump/motor 31 reaches zero, the clutches27, 29, 37, 40 are switched as shown in TABLE 1 by the clutch pressurecontrol valves 53, 54, 56, 57 that operate in response to instructionsignals from the controller 50.

First, in the initial state where vehicle speed is zero, the firstclutch 27 is turned OFF as shown in Table 2 to disengage the carrier 13of the second planetary gear train 6 from the ring gear 10 of the firstplanetary gear train 5, whereas the second clutch 29 is turned ON asshown in Table 2 to bring the carrier 13 of the second planetary geartrain 6 into engagement with the fixed end 28, so that the input splitmode is established. In the above initial state, vehicle speed is equalto or lower than V1 and therefore the third clutch 37 is turned ON andthe fourth clutch 40 is turned OFF as shown in TABLE 2, so that thethird pump/motor 31 is connected to the first pump/motor 16 in parallel(range A).

In the range A, the power of the engine 2 is input to the sun gear 7 ofthe first planetary gear train 5 and this power is output from thecarrier 9 to the sun gear 11 of the second planetary gear train 6through the sleeve shaft 23. The power input to the sun gear 7 istransmitted from the planetary gears 8 to the ring gear 10 and thenoutput to the second pump/motor 20 that serves as a pump, through thethird gear 19 and the fourth gear 22. The power output to the secondpump/motor 20 is transmitted through the hydraulic pipeline 30 to thefirst pump/motor 16 that serves as a motor and transmitted through thehydraulic pipeline 32 to the third pump/motor 31 that serves as a motor.Further, the rotational power of the first pump/motor 16 is output fromits output shaft 17 to the sun gear 11 of the second planetary geartrain 6 through the second gear 18, the first gear 15, the carrier 9 andthe sleeve shaft 23. The rotational power of the third pump/motor 31 isoutput from its output shaft 41 to the sun gear 11 of the secondplanetary gear train 6 through the eleventh gear 42, the seventh gear35, the first shaft 33, the eighth gear 36, the first gear 15, thecarrier 9 and the sleeve shaft 23. In this way, the power output to thesun gear 11 of the second planetary gear train 6 is transmitted from theplanetary gears 12 to the output shaft 25 through the ring gear 14, thefifth gear 24 and the sixth gear 26 and becomes the rotational power ofthe output shaft 25. The third pump/motor 31 connected to the firstpump/motor 16 in parallel plays the role of assisting the motoringfunction of the first pump/motor 16.

It should be noted that the second embodiment has been described in thecontext of the transmission system 1A that is applied to a constructionvehicle. Therefore, the second embodiment is designed, similarly to thefirst embodiment, such that the displacement of the second pump/motor 20during the initial state where vehicle speed is zero is not set to zeroin order to obtain traction force by generating torque on the outputshaft 25 even when vehicle speed is zero (see FIG. 4( c)). A detainedexplanation will be skipped herein because the second embodiment issimilar to the first embodiment in this respect.

As shown in Table 2, in the vehicle speed range B exceeding V1, thethird clutch 37 is turned OFF and the fourth clutch 40 is turned ON sothat the third pump/motor 31 is connected to the second pump/motor 20 inparallel. In doing so, the displacement of the third pump/motor 31 iszero at the instant when vehicle speed changes from the range A to V1.At the same time, the teeth numbers of the gears to be connected to thefirst to third pumps/motors 16, 20, 31 are properly determined, wherebythe relative rotational speed difference between the tenth gear 39 andthe second shaft 34 in this condition, that is, the relative rotationalspeed difference of the fourth clutch 40 in the idle condition can bemade to be zero. However, the third pump/motor 31 and the eleventh gear42, seventh gear 35 and tenth gear 39 which are connected to the thirdpump/motor 31 start to decrease in speed owing to their own friction orthe like because the driving force for rotating the third pump/motor 31is cut off on and after the instant when vehicle speed reaches V1 andthe third clutch 37 is turned OFF. As a result, when turning the fourthclutch 40 ON to connect the third pump/motor 31 to the second pump/motor20 in parallel, a relative rotational speed difference occurs betweenthe tenth gear 39 and the second shaft 34. As a clutch mechanism forabsorbing such a rotational speed difference, hydraulic multiple discclutches or synchromesh mechanisms, which are simpler in structure andless costly than hydraulic multiple disc clutches, may be employed. Inaddition, in cases where the rotational speed difference between thegear 39 and the shaft 34 can be made to be zero by the techniquedescribed later, dog clutches that are much simpler in structure can beemployed.

In the range B, the power of the engine 2 is input to the sun gear 7 ofthe first planetary gear train 5 and this power is output from thecarrier 9 to the sun gear 11 of the second planetary gear train 6through the sleeve shaft 23. The power input to the sun gear 7 is outputfrom the planetary gears 8 to the third gear 19 through the ring gear10. The power output to the third gear 19 is output through the fourthgear 22 to the second pump/motor 20 that serves as a pump and outputthrough the ninth gear 38, the second shaft 34, the tenth gear 39, theseventh gear 35 and the eleventh gear 42 to the third pump/motor 31 thatserves as a pump. The power output to the second pump/motor 20 and thepower output to the third pump/motor 31 are transmitted to the firstpump/motor 16 through the hydraulic pipelines 30 and 32, respectively.The rotational power of the first pump/motor 16 is output from itsoutput shaft 17 to the sun gear 11 of the second planetary gear train 6through the second gear 18, the first gear 15, the carrier 9 and thesleeve shaft 23. In this way, the power output to the sun gear 11 of thesecond planetary gear train 6 is transmitted from the planetary gears 12to the output shaft 25 through the ring gear 14, the fifth gear 24 andthe sixth gear 26 and becomes the rotational power of the output shaft25. In the range B, the third pump/motor 31 accordingly plays the roleof assisting the pumping function of the second pump/motor 20. In therange B, only the object which the third pump/motor 31 assists differsfrom that of the range A but the mode of the transmission system remainsin the input split mode.

As shown in Table 2, in a vehicle speed range C exceeding V2, the firstclutch 27 is turned ON to engage the carrier 13 of the second planetarygear train 6 with the ring gear 10 of the first planetary gear train 5whereas the second clutch 29 is turned OFF to disengage the carrier 13of the second planetary gear train 6 from the fixed end 28, so that thecompound split mode is established. That is, the vehicle speed V2 is themode switching point at which shifting between the input split mode andthe compound split mode is effected. The vehicle speed V2 is also thelow speed side direct point at which the displacement of the firstpump/motor 16 becomes zero and the power of the engine is alltransmitted to the output shaft 25 through the mechanical transmissionsection alone. In the second embodiment, at the vehicle speed V2, therotational speeds of the second pump/motor 20 and the third pump/motor31 are both zero and the rotational speed of the ring gear 10 of thefirst planetary gear train 5 is equal to the rotational speed of thecarrier 13 of the second planetary gear train 6 irrespective of thestates of the third clutch 37 and the fourth clutch 40. Therefore, a dogclutch simple in structure can be used as the first clutch 27 and thesecond clutch 29.

In the range C, the power of the engine 2 is input to the sun gear 7 ofthe first planetary gear train 5 and this power is output from thecarrier 9 to the sun gear 11 of the second planetary gear train 6through the sleeve shaft 23. The power input to the sun gear 7 is alsooutput from the carrier 9 to the first pump/motor 16 that serves as apump, through the first gear 15 and the second gear 18. The power outputto the first pump/motor 16 is then transmitted through the hydraulicpipeline 30 to the second pump/motor 20 that serves as a motor andtransmitted through the hydraulic pipeline 32 to the third pump/motor 31that serves as a motor. The rotational power of the second pump/motor 20is output from its output shaft 21 to the carrier 13 of the secondplanetary gear train 6 through the fourth gear 22, the third gear 19 andthe ring gear 10, whereas the rotational power of the third pump/motor31 is output from its output shaft 41 to the carrier 13 of the secondplanetary gear train 6 through the eleventh gear 42, the seventh gear35, the tenth gear 39, the second shaft 34, the ninth gear 38, the thirdgear 19 and the ring gear 10. The power output to the sun gear 11 of thesecond planetary gear train 6 is output to the ring gear 14 through theplanetary gears 12, whereas the power output to the carrier 13 of thesecond planetary gear train 6 is output to the ring gear 14 through theplanetary gears 12. These powers join together at the ring gear 14 to betransmitted to the output shaft 25 through the fifth gear 24 and thesixth gear 26 and thus become the rotational power of the output shaft25. In the range C, the third pump/motor 31 plays the role of assistingthe second pump/motor 20 in performing its motoring functioncontinuously from the range B.

As shown in Table 2, in a vehicle speed range D exceeding V3, the thirdclutch 37 is turned ON and the fourth clutch 40 is turned OFF, therebyconnecting the third pump/motor 31 to the first pump/motor 16 inparallel. In so doing, at the instant when vehicle speed changes fromthe range C to V3, the displacement of the third pump/motor 31 is zero.At the same time, the teeth numbers of the gears connected to the firstto third pumps/motors 16, 20, 31 are properly determined, thereby makingthe relative rotational speed difference between the tenth gear 39 andthe second shaft 34 in this condition, i.e., the relative rotationalspeed difference of the fourth clutch 40 in the idle condition be zero.However, the third pump/motor 31 and the eleventh gear 42, seventh gear35 and tenth gear 39 which are connected to the third pump/motor 31start to decrease in speed owing to their own friction or the likebecause the driving force for rotating the third pump/motor 31 is cutoff on and after the instant when vehicle speed reaches V3 and thefourth clutch 40 is turned OFF. As a result, when turning the thirdclutch 37 ON to connect the third pump/motor 31 to the first pump/motor16 in parallel, a relative rotational speed difference occurs betweenthe eighth gear 36 and the first shaft 33. As a clutch mechanism forabsorbing such a rotational speed difference, hydraulic multiple discclutches or synchromesh mechanisms that are simpler in structure andless costly than hydraulic multiple disc clutches may be employed. Inaddition, in cases where the rotational speed difference between thegear 36 and the shaft 33 can be made to be zero by the techniquedescribed later, dog clutches that are much simpler in structure can beemployed.

In the range D, the power of the engine 2 is input to the sun gear 7 ofthe first planetary gear train 5 and this power is output from theplanetary gears 8 to the carrier 13 of the second planetary gear train 6through the ring gear 10. The power input to the sun gear 7 is output tothe carrier 9 and the power output to the carrier 9 is then outputthrough the sleeve shaft 23 to the sun gear 11 of the second planetarygear train 6 and to the first gear 15. The power output to the firstgear 15 is output through the second gear 18 to the first pump/motor 16that serves as a pump and is also output through the eighth gear 36, thefirst shaft 33, the seventh gear 35 and the eleventh gear 42 to thethird pump/motor 31 that serves as a pump. The power output to the firstpump/motor 16 and the power output to the third pump/motor 31 are thentransmitted to the second pump/motor 20 that serves as a motor, throughthe hydraulic pipeline 30 and the hydraulic pipeline 32, respectively.The rotational power of the second pump/motor 20 is output from itsoutput shaft 21 to the carrier 13 of the second planetary gear train 6through the fourth gear 22, the third gear 19 and the ring gear 10. Inthis way, the power output to the sun gear 11 of the second planetarygear train 6 is output to the ring gear 14 through the planetary gears12, and the power output to the carrier 13 of the second planetary geartrain 6 is also output to the ring gear 14 through the planetary gears12. These powers join together at the ring gear 14 to be output to theoutput shaft 25 through the fifth gear 24 and the sixth gear 26 andaccordingly become the rotational power of the output shaft 25. In therange D, the third pump/motor 31 accordingly plays the role of assistingthe pumping function of the first pump/motor 16. In the range D, onlythe object which the third pump/motor 31 assists differs from that ofthe range C but the mode of the transmission system remains in thecompound split mode.

At the vehicle speed V4, the rotational speeds of the first pump/motor16 and the third pump/motor 31 reach zero and the displacement of thesecond pump/motor 20 becomes zero. At that time, the first pump/motor 16and the third pump/motor 31 are brought into a stopped state and thesecond pump/motor 20 runs idle so that no oil flows in the hydraulicpipelines 30, 32, causing no hydraulic power transmission. As a result,vehicle speed becomes the high speed side direct point at which thepower of the engine 2 is all transmitted through the mechanicaltransmission section alone.

The transmission system 1A of the second embodiment has the sameoperational effect as of the transmission system 1 of the firstembodiment since they have the same basic configuration. According tothe transmission system 1A of the second embodiment, the thirdpump/motor 31 can be selectively placed on the first pump/motor 16 sideor the second pump/motor 20 side whereby the pump/motor 31 can be usedfor assisting the function of either pump/motor 16 or 20. This allowsuse of pumps/motors of smaller capacity as the pumps/motors 16, 20, 31.Herein, it is desirable to set the displacement capacities of the firstpump/motor 16, the second pump/motor 20 and the third pump/motor 31 tosubstantially the same value. This makes it possible to employpumps/motors having substantially the same specification for them andtherefore achieve improved interchangeability.

Provision of the state in which the third clutch 37 and the fourthclutch 40 are turned ON at the same time when shifting the thirdpump/motor 31 from one side to the other according to the vehicle speedsV1, V3 allows the vehicle speeds V1, V3 to be a direct point at whichengine power is all transmitted through the mechanical transmissionsection only. In this case, four direct points can be provided in total,including the above-described low speed side direct point (=the modeswitching point) and high speed side direct point.

Next, there will be explained the operation of a switching mechanism 44during acceleration from a low vehicle speed to a high vehicle speed, byway of an example in which vehicle speed changes from V1 to the abovedirect state. When vehicle speed is lower than V1, the third pump/motor31 is connected to the first pump/motor 16 in parallel. That is, thethird clutch 37 is ON whereas the fourth clutch 40 is OFF. The vehicleis accelerated from this condition and at the instant when vehicle speedhas reached V1, the relative rotational speed of the fourth clutch 40 inits OFF state, that is, the relative rotational speed difference betweenthe ninth gear 38 and the tenth gear 39 becomes nil. Specifically, atthe moment when vehicle speed has reached V1 from a lower speed, thefourth clutch 40 can be engaged in a condition where no relativerotational speed difference exists. Therefore, dog clutches that aresimpler in structure and less costly than hydraulic multiple discclutches and synchromesh mechanisms may be employed for the third clutch37 and the fourth clutch 40. In addition, if the vehicle is driven at adirect point, the hydraulic pipeline 30, which connects the firstpump/motor 16 to the second pump/motor 20, is brought into acommunicating state by means of a communicating valve 58 provided in thehydraulic pipeline 30. This enables it to completely cut off the powertransmission by oil pressure so that the power loss in the hydraulicpath can be minimized and the efficiency of the transmission system canbe increased.

Third Embodiment

FIG. 6 is a schematic structural diagram of a transmission systemaccording to a third embodiment of the invention. FIG. 7 shows operatingcharacteristic graphs of the transmission system according to the thirdembodiment. The third embodiment is associated with an example where thetransmission system 1A of the second embodiment is additionally providedwith a switching valve 43 for effecting switching such that the flow ofpressure oil to the third pump/motor 31 is constantly directed in afixed direction. Therefore, parts similar or corresponding to those ofthe second embodiment are identified with the same reference numeralsand a detailed description thereof is omitted herein.

The third embodiment provides a transmission system 1B characterized bya mono-directional pump/motor used as the third pump/motor 31 (see FIG.7( d)). In some cases, the rotating direction of the third pump/motor 31when connected to the first pump/motor 16 side is opposite to that ofthe third pump/motor 31 when connected to the second pump/motor 20 side.Therefore, the transmission system 1A of the second embodiment has touse a bi-directional pump/motor as the third pump/motor 31 (see FIG. 4(d)). In contrast with this, the third embodiment can use amono-directional pump/motor as the third pump/motor 31 and attain costreduction, by employing such a configuration that the switching valve 43is provided in the hydraulic pipeline 32 and shifted between positions Aand B in accordance with whether the third pump/motor 31 is connected tothe first pump/motor 16 side or the second pump/motor 20 side.

Fourth Embodiment

FIG. 8 is a schematic structural diagram of a transmission systemaccording to a fourth embodiment of the invention. In this embodiment,parts similar or corresponding to those of the first embodiment areidentified with the same reference numerals and a detailed descriptionthereof is omitted herein.

The fourth embodiment provides a transmission system 1C having the inputshaft 4 that inputs power from the engine 2 through the forward/reverseswitching mechanism 3 and the first planetary gear train 5 and secondplanetary gear train 6 that are coaxially aligned on the input shaft 4.

The first planetary gear train 5 is composed of the sun gear 7 securedto the input shaft 4; the plurality of planetary gears 8 in meshingengagement with the outer periphery of the sun gear 7; the carrier 9 forsupporting the shaft of the planetary gears 8; and the ring gear 10 inmeshing engagement with the outer periphery of the group of theplanetary gears 8. The second planetary gear train 6 is composed of thesun gear 11; the plurality of planetary gears 12 in meshing engagementwith the outer periphery of the sun gear 11; the carrier 13 forsupporting the shaft of the planetary gears 12; and the ring gear 14 inmeshing engagement with the outer periphery of the group of planetarygears 12.

The first gear 15 is integrally coupled to the carrier 9 of the firstplanetary gear train 5 and meshingly engaged with the fourth gear 22secured to the output shaft 21 of the second pump/motor 20. The thirdgear 19 is integrally coupled to the ring gear 10 of the first planetarygear train 5 and meshingly engaged with the second gear 18 secured tothe output shaft 17 of the first pump/motor 16. The ring gear 10 of thefirst planetary gear train 5 and the sun gear 11 of the second planetarygear train 6 are coupled to each other by a sleeve shaft 61 that isrotatably supported on an intermediate output shaft 60. The fifth gear24 is integrally coupled to the ring gear 14 of the second planetarygear train 6 and meshingly engaged with the sixth gear 26 secured to theoutput shaft 25 of the fifth gear 24.

The transmission system 1C is provided with the first clutch 27 forconnecting and disconnecting the carrier 13 of the second planetary geartrain 6 to and from the carrier 9 of the first planetary gear train 5through the intermediate output shaft 60 and provided with the secondclutch 29 for connecting and disconnecting the carrier 13 of the secondplanetary gear train 6 to and from the fixed end 28.

Reference is made to FIG. 9 and Table 3 to describe a case where vehiclespeed is accelerated from zero while keeping the rotational speed of theengine 2 input to the input shaft 4 constant in the transmission system1C of the fourth embodiment. FIG. 9 shows operating characteristicgraphs of the transmission system of the fourth embodiment. FIG. 9( a)is a graph showing changes in the rotational speed of each pump/motorrelative to vehicle speed; FIG. 9( b) is a graph showing changes in thedisplacement of the first pump/motor relative to vehicle speed; and FIG.9( c) is a graph showing changes in the displacement of the secondpump/motor relative to vehicle speed. Table 3 shows the operatingcondition (ON (connected)/OFF (disconnected) state) of each clutch inrelation to vehicle speed ranges. In FIG. 9( a), solid line representschanges in the rotational speed of the first pump/motor relative tovehicle speed whereas dotted line represents changes in the rotationalspeed of the second pump/motor relative to vehicle speed.

TABLE 3 Vehicle Speed Range A Vehicle Speed Range B First Clutch OFF ONSecond Clutch ON OFF

The displacements of the pumps/motors 16, 20 are controlled according tovehicle speed as shown in FIGS. 9( b), 9(c), by means of the first andsecond pump/motor displacement control systems 51, 52, respectively,that operate in response to instruction signals from the controller 50.As a result, the rotational speeds of the pumps/motors 16, 20 vary asshown in FIG. 9( a). At the vehicle speed V2 which serves as theswitching point and at which the displacement of the first pump/motor 16becomes zero, the first and second clutches 27, 29 are switched as shownin Table 3 by the first and second clutch pressure control valves 53,54, respectively, that operate in response to instruction signals fromthe controller 50.

As shown in Table 3, in the vehicle speed range A not higher than thevehicle speed V2, the first clutch 27 is turned OFF to disengage thecarrier 13 of the second planetary gear train 6 from the carrier 9 ofthe first planetary gear train 5, whereas the second clutch 29 is turnedON to engage the carrier 13 of the second planetary gear train 6 withthe fixed end 28, thereby establishing the input split mode. At thattime, the power of the engine 2 is input to the sun gear 7 of the firstplanetary gear train 5 and this power is transmitted from the planetarygears 8 to the ring gear 10 and output to the sun gear 11 of the secondplanetary gear train 6 through the sleeve shaft 61. The power input tothe sun gear 7 is transmitted from the carrier 9 to the secondpump/motor 20 that serves as a pump, through the first gear 15 and thefourth gear 22. The power output to the second pump/motor 20 istransmitted through the hydraulic pipeline 30 to the first pump/motor 16that serves as a motor. The rotational power of the first pump/motor 16is output from the output shaft 17 to the sun gear 11 of the secondplanetary gear train 6 through the second gear 18, the third gear 19,the ring gear 10 and the sleeve shaft 61. In this way, the power outputto the sun gear 11 of the second planetary gear train 6 is transmittedfrom the planetary gears 12 to the output shaft 25 through the ring gear14, the fifth gear 24 and the sixth gear 26 and becomes the rotationalpower of the output shaft 25.

The fourth embodiment has been described in the context of thetransmission system C applied to a construction vehicle. Similarly tothe first embodiment, the displacement of the second pump/motor 20 isnot set to zero in the initial state in which vehicle speed is zero, inorder to allow the output shaft 25 to generate torque to producetraction force even when vehicle speed is zero (see FIG. 9( c)). Adetained explanation will be skipped because the fourth embodiment doesnot differ from the first embodiment in this respect.

As shown in Table 3, in the vehicle speed range B exceeding V2, thefirst clutch 27 is turned ON to engage the carrier 13 of the secondplanetary gear train 6 with the carrier 9 of the first planetary geartrain 5, whereas the second clutch 29 is turned OFF to disengage thecarrier 13 of the second planetary gear train 6 from the fixed end 28,thereby establishing the compound split mode. That is, the vehicle speedV2 is the mode switching point at which shifting between the input splitmode and the output split mode is effected. The vehicle speed V2 is alsothe low speed side direct point at which the displacement of the firstpump/motor 16 is zero and engine power is all transmitted to the outputshaft 25 through the mechanical transmission section alone. At thevehicle speed V2, the rotational speed of the second pump/motor 20 iszero and the rotational speed of the carrier 9 of the first planetarygear train 5 is equal to that of the carrier 13 of the second planetarygear train 6. Therefore, dog clutches of simple structure can be usedfor the first clutch 27 and the second clutch 29.

In the vehicle speed range B, the power of the engine 2 is input to thesun gear 7 of the first planetary gear train 5 and this power is outputfrom the planetary gears 8 to the ring gear 10. The power transmitted tothe ring gear 10 is output to the sun gear 11 of the second planetarygear train 6 through the sleeve shaft 61 and output to the firstpump/motor 16 that serves as a pump, through the third gear 19 and thesecond gear 18. The power output to the first pump/motor 16 is thentransmitted through the hydraulic pipeline 30 to the second pump/motor20 that serves as a motor. The rotational power of the second pump/motor20 is transmitted from its output shaft 21 to the carrier 13 of thesecond planetary gear train 6 through the fourth gear 22, the first gear15, the carrier 9 and the intermediate output shaft 60. In this way, thepower output to the sun gear 11 of the second planetary gear train 6 isoutput to the ring gear 14 through the planetary gears 12, and the poweroutput to the carrier 13 of the second planetary gear train 6 is alsooutput to the ring gear 14 through the planetary gears 12. These powersjoin together at the ring gear 14 to be transmitted to the output shaft25 through the fifth gear 24 and the sixth gear 26 and become therotational power of the output shaft 25.

At the vehicle speed V4, the rotational speed of the first pump/motor 16reaches zero while the displacement of the second pump/motor 20 becomeszero. At that time, the second pump/motor 20 runs idle with the firstpump/motor 16 being stopped, and no oils flow in the hydraulic pipeline30 so that no power is transmitted by oil pressure. Thus, vehicle speedbecomes the high speed side direct point at which the power of theengine 2 is all transmitted through the mechanical transmission sectionalone.

In the transmission system 1C of the fourth embodiment, shifting betweenthe input split mode and the compound split mode is effected on thebasis of the vehicle speed (V2: mode switching point) at which therotational speed of the second pump/motor becomes zero. Specifically, ifpresent vehicle speed is in a vehicle speed range below the modeswitching point, the carrier 13 of the second planetary gear train 6 isdisengaged from the carrier 9 of the first planetary gear train 5 by thefirst clutch 27, while engaging the carrier 13 of the second planetarygear train 6 with the fixed end 28 by the second clutch 29, whereby theinput split mode is established. In the input split mode in whichvehicle speed is zero, the rotational speed of the first pump/motor 16is zero and the rotational speed of the second pump/motor 20 at acertain input rotational speed is uniquely determined in accordance withthe tooth number ratio of the first planetary gear train 5. At the highspeed side direct point that provides the compound split mode (i.e.,when the rotational speed of the first pump/motor 16 is zero), therotational speed of the second pump/motor 20 at the certain inputrotational speed is uniquely determined in accordance with the toothnumber ratio of the first planetary gear train 5. Specifically, as shownin FIG. 9( a), the rotational speed of the second pump/motor 20 becomes+N1 at both zero vehicle speed and the high speed side direct point V4.Thus, the same rotational speed and same rotating direction can be setfor these modes. Therefore, whatever vehicle speed the mode switchingpoint is set to, the applicable rotational speed range of the secondpump/motor 20 in the input split mode and the applicable rotationalspeed range of the second pump/motor 20 in the compound split mode canbe evenly balanced, so that a pump/motor having smaller capacity thanthe prior art can be used as the second pump/motor 20. In addition,since the rotating direction in the input split mode does not differfrom the rotating direction in the compound split mode, amono-directional pump/motor can be used as the second pump/motor 20.

Fifth Embodiment

FIG. 10 is a schematic structural diagram of a transmission systemaccording to a fifth embodiment of the invention. The transmissionsystem 1D shown in FIG. 10 is configured to have the third pump/motor 31that is connected to the transmission system 1C of the fourth embodimentthrough a switching mechanism 44A (that is basically the same as theswitching mechanism 44). This third pump/motor 31 is connected to thehydraulic pipeline 30 through the hydraulic pipeline 32 such that it canbe selectively connected to the first pump/motor 16 side or to thesecond pump/motor 20 side. In this way, the third pump/motor 31 isdesigned to play the role of assisting the function of either thepump/motor 16 or the pump/motor 20 at any time. According to the fifthembodiment, pumps/motors having smaller capacity can be used as thepumps/motors 16, 20, 31 like the second embodiment.

Sixth Embodiment

FIG. 11 is a schematic structural diagram of a transmission systemaccording to a sixth embodiment of the invention. The transmissionsystem 1E shown in FIG. 11 is a modification of the transmission system1D of the fifth embodiment, in which the switching valve 43 is providedin the hydraulic pipeline 32 to thereby enable use of a mono-directionalpump/motor as the third pump/motor 31. The sixth embodiment achievesfurther cost reduction like the third embodiment.

Seventh Embodiment

FIG. 12 is a schematic structural diagram of a transmission systemaccording to a seventh embodiment of the invention. The transmissionsystem 1F shown in FIG. 12 is one example of electro-mechanicaltransmissions in which the pumps/motors 16, 20 of the first embodimentare replaced with generators/motors 16A, 20A. In this case, drivingcontrol of the generators/motors 16A, 20A is performed by a controller50A connected to a battery 65. Herein, the controller 50A is formed suchthat an inverter circuit is incorporated into the above-describedcontroller 50. It is desirable to interpose an electric storage devicebetween the generator and the motor, for storing electric energy (thisis also applied to the eighth to tenth embodiments described below).According to the seventh embodiment, a compact generator/motor havingsmaller capacity than the prior art can be used as the secondgenerator/motor 20A, like the first embodiment.

Eighth Embodiment

FIG. 13 is a schematic structural diagram of a transmission systemaccording to an eighth embodiment of the invention. The transmissionsystem 1G shown in FIG. 13 is one example of electro-mechanicaltransmissions in which the pumps/motors 16, 20, 31 of the secondembodiment are replaced with generators/motors 16A, 20A, 31A. In thiscase, driving control of the generators/motors 16A, 20A, 31A isperformed by the controller 50A connected to the battery 65. Accordingto the eighth embodiment, compact generators/motors having smallercapacity can be used as the generators/motors 16A, 20A, 31A like thesecond embodiment.

Ninth Embodiment

FIG. 14 is a schematic structural diagram of a transmission systemaccording to a ninth embodiment of the invention. The transmissionsystem 1H shown in FIG. 14 is one example of electro-mechanicaltransmissions in which the pumps/motors 16, 20 of the fourth embodimentare replaced with generators/motors 16A, 20A. In this case, drivingcontrol of the generators/motors 16A, 20A is performed by the controller50A connected to the battery 65. According to the ninth embodiment, acompact generator/motor having smaller capacity than the prior art canbe used as the second generator/motor 20A like the fourth embodiment.

Tenth Embodiment

FIG. 15 is a schematic structural diagram of a transmission systemaccording to a tenth embodiment of the invention. The transmissionsystem 1I shown in FIG. 15 is one example of electro-mechanicaltransmissions in which the pumps/motors 16, 20, 31 of the fifthembodiment are replaced with generators/motors 16A, 20A, 31A. In thiscase, driving control of the generators/motors 16A, 20A, 31A isperformed by the controller 50A connected to the battery 65. Accordingto the tenth embodiment, compact generators/motors having smallercapacity can be used as the generators/motors 16A, 20A, 31A like thefifth embodiment.

Although the first to tenth embodiments described above may usehydraulic clutches, synchromesh mechanisms and dog clutches for thefirst to fourth clutches, the first to fourth clutches are not limitedto these clutches but various types of clutch mechanisms such aselectromagnetic clutches are applicable.

1. A transmission system comprising: an input shaft; an output shaft; amechanical transmission section; and a hydraulic transmission section;wherein the mechanical transmission section and the hydraulictransmission section are interposed between the input shaft and theoutput shaft; wherein the hydraulic transmission section includes aplurality of pumps/motors which are connectable to each other through ahydraulic circuit, the plurality of pumps/motors including a firstpump/motor and a second pump/motor; wherein the mechanical transmissionsection has a first planetary gear train and a second planetary geartrain; wherein a sun gear of the first planetary gear train iscontinuously connected to the input shaft, a carrier of the firstplanetary gear train is continuously connected to a sun gear of thesecond planetary gear train and is connected to the first pump/motor, aring gear of the first planetary gear train is connected to the secondpump/motor, and a ring gear of the second planetary gear train isconnected to the output shaft; wherein a first clutch is provided forengaging and disengaging a carrier of the second planetary gear trainand the ring gear of the first planetary gear train with and from eachother; and wherein a second clutch is provided for engaging anddisengaging the carrier of the second planetary gear train and a fixedend with and from each other.
 2. The transmission system according toclaim 1, wherein a speed ratio of a low speed side direct point at whicha rotational speed of the second pump/motor becomes zero to a high speedside direct point at which a rotational speed of the first pump/motorbecomes zero is set to 3 to
 4. 3. The transmission system according toclaim 1, wherein: each of the carrier of the first planetary gear trainand the carrier of the second planetary gear train includes a planetarygear.
 4. The transmission system according to claim 1, wherein each ofthe first clutch and the second clutch comprises one of a dog clutch anda synchromesh mechanism.